The present invention relates to an energy storage system for a motor vehicle including an electric machine, which can be driven by an internal combustion engine and can be operated at least as a generator; a vehicle electrical system battery, which is connected to the electric machine in parallel; and a controller for actuating the electric machine such that a defined desired voltage of the generator is specified to the electric machine as a function of the presence or absence of a boost or braking operation.
German patent document DE 100 46 631 A1 discloses an energy storage system including a generator, a battery of the vehicle electrical system, as well as a controller with a generator regulator and generator control unit for specifying a desired generator voltage. The desired voltage is specified as a function of the driving condition variables such that, in the braking or boost operating mode of the vehicle, electric energy is fed into the vehicle electrical system. The method for regulating the generator voltage is carried out such that, depending on the specified changeover conditions, there is a changeover between a recuperation readiness mode and a recovery mode for the battery of the vehicle electrical system. For example, the charge throughput through the battery since the last changeover from the recovery mode into the recuperation readiness mode may be used as the changeover condition.
An object of the invention is to improve an energy storage system of the class described above. In particular, the invention shall provide an energy storage system, which is easy to construct and, with respect to the efficiency of energy accumulation, increases the energy gained by recuperation.
The invention is based on the knowledge that the recuperation capacity of known energy storage systems is limited by the capacity of a battery of the vehicle electrical system to consume high power. In the event that the state of charge is 70%, a conventional battery of the vehicle electrical system can consume a maximum current in the range of 50 A. A double layer capacitor, which may be used in the invention, can consume or provide current ranging up to 200 A. The present invention solves this problem.
According to the present invention, the energy storage system includes not only an electric machine for generating electric energy, a vehicle electrical system battery, which is connected to the electric machine in parallel, and a controller for controlling the electric machine via adjustment of the generator output voltage or rather the generator output current, but also a capacitor unit, which is connected in parallel to the electric machine and the battery of the vehicle electrical system. The controller is configured, according to the invention, such that in the event that the vehicle is in the boost or braking operating mode, the battery of the vehicle electrical system is charged with a maximum charging current (battery charging current/generator current). In all other cases, the battery of the vehicle electrical system is charged by a charge current that is adjusted as a function of the charge state of the battery of the vehicle electrical system. Beyond the boost and braking operating mode, a distinction is made among the following three cases.
In the event that the state of charge exceeds a pre-defined first charge threshold value, a targeted discharging of the battery of the vehicle electrical system is carried out by regulating the generator current such that the result is a pre-defined minimum battery charging current.
In the event that the state of charge falls below a pre-defined second charge threshold value, the battery of the vehicle electrical system is charged by regulating the generator current in such a way that the result is a pre-defined maximum battery charging current.
In the event that the state of charge is equivalent to the specified state of charge, the generator current is regulated to guarantee that no and/or the lowest possible (charging or discharging) current flows through the battery of the vehicle electrical system. The first charge threshold value may be different from the second charge threshold value. Therefore, for the third case the result is that the state of charge is equivalent to the specified state of charge, when the latter is between the two specified threshold values of the charge states. Outside the boost or braking operating mode, the battery of the vehicle electrical system may be charged by means of a pre-defined map.
In the case of a boost or braking operation, a changeover is made from the map control to an operating mode (generator operating mode), in which the electric machine charges the battery or rather the capacitor, connected to the battery in parallel, with a specified maximum current. In an exemplary embodiment of the invention, the controller is configured such that the vehicle speed is monitored, and if a specified driving speed is not reached, the capacitor is charged up to a pre-defined charge value.
In another exemplary embodiment of the invention, the controller is configured such that the vehicle speed is monitored, and if a specified driving speed is exceeded, or if a specified driving speed is exceeded for a specified period of time, the capacitor is discharged to a pre-defined charge value. Exceeding the speed implies a future braking operation within a defined period of time; and for this reason the capacitor is discharged as far as to a pre-defined value (and thereby reducing the load on the electric machine) in order to consume sufficient energy again during a forthcoming braking operation.
Furthermore, the controller may be configured such that, in the event that the motor vehicle is switched off in a detectable manner (e.g., ignition OFF and/or the vehicle locked and/or the driver's seat or rather the seat is not occupied), the capacitor unit is separated electrically from the vehicle electrical system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.